Sodium and Hydrogen/Deuterium DoubletsA Fabry-Perot interferometer is used to separate the spectral lines of hydrogen from those of deuterium. The expected interference fringe pattern from a single spectral line is a "bullseye" of concentric light and dark rings. If another spectral line is present, a series of independent darker and lighter concentric rings can be seen. The appearance of these rings depends upon the separation of the elements of the interferometer, i.e., the thickness of the air film between the surfaces. Appropriate data can then yield the separation of the spectral wavelengths. Since the measurement of the H/D doublet requires a well-aligned interferometer, it is good practice to align the spectrometer and measure the sodium doublet as preparation for the H/D measurement. The recommended experimental procedure below offers some helpful steps in the process. Measurement of the small H/D doublet requires high resolvance with the Fabry-Perot interferometer, so the following steps are recommended. 1. Carefully align the mirrors with the aid of a helium-neon laser. 2. Measure the separation of the sodium doublet. 3. Carefully optimize the fringes for the H/D doublet and make several measurements of the wavelength separation.
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HyperPhysics***** Class Home ***** Physics 3901 | Go Back |
Equipment NeededFabry-Perot interferometerHeavy metal base for mounting Sodium light and helium-neon laser for test sources Hydrogen and deuterium spectrum tubes Spectrum power supply. Colored filters, red and blue (the red hydrogen interference filter is good for the red lines) Lab jacks Small lamp so room can be kept dark magnifying lens for viewing fringes Operating instructions for the M4 Spectrometer, Cenco Company Bulletin GB163-73A from Gaertner, which describes their Modular Interferometer (the one used in this experiment) Ground glass to aid focusing of line source (spectrum tube) Masking tape |
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HyperPhysics***** Physics 3901 ***** H/D Doublet | Go Back |
Sodium Doublet MeasurementMeasurement of the sodium doublet with the Fabry-Perot interferometer is good preparation for the more difficult hydrogen/deuterium doublet measurement. Some of the steps involved are: 1. Align the mirrors with use of He-Ne laser. 2. Place sodium lamp on side opposite micrometer screw. This extended source can be used directly, so that no condensing lens or ground glass screen is needed. Adjust the interferometer until you can see the bullseye interference pattern. 3. The doublet produces two bullseye patterns which overlap. One line of the doublet is twice as intense as the other, so that pattern is brighter. By moving the micrometer screw in one direction only to avoid the backlash problem, find a point where the dimmer bullseye lines are halfway between the brighter ones. Make a measurement. 4. Being carefull to turn the micrometer consistently in the same direction, advance the micrometer until you reach the next point at which the dimmer bullseye lines are at the halfway point and make another measurement. 5. Using the fact that you have now advanced one set of fringes one full order compared to the other, calculate the wavelength separation. |
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HyperPhysics***** Physics 3901 ***** H/D Doublet | Go Back |
Sodium Doublet on Fabrey-PerotThe image below shows the appearance of the interference fringes of the sodium doublet produced by Fabry-Perot optics. This is the appearance at one position.
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